1. Technical Field
The present invention relates to a field effect transistor.
2. Related Art
Field effect transistors are widely used as unit elements of integrated circuits for semiconductor memories, high-frequency signal amplifying elements, elements for driving display elements such as liquid crystals, and the like. Field effect transistors that are made to be thin-filmed in particular are called thin film transistors (TFTs). Further, a silicon TFT, that has an active layer formed from amorphous silicon that can be formed over a large surface area, is used in flat panel displays.
The development of TFTs, that use In—Ga—Zn—O (IGZO) oxide semiconductors in the active layer (channel layer) instead of this amorphous silicon, has be carried out actively in recently years. With oxide semiconductors, film formation at a low temperature is possible, and oxide semiconductors exhibit higher mobility than amorphous silicon, and further, are transparent with respect to visible light. Therefore, a flexible and transparent TFT can be formed on a substrate such as a plastic plate or a film or the like.
By the way, in such field effect transistors, there are cases in which, due to a large current flowing when a high voltage is applied (at times of driving), Joule heat is generated and the temperature of the active layer rises locally. The generation of heat that arises at times of driving in this way is one cause that brings about deterioration in driving (deterioration in reliability).
Thus, Japanese Patent Application Laid-Open No. 2003-131588 proposes, in order to prevent generated heat that arises at the time of driving from being stored, forming a film, that has higher thermal conductivity than a glass substrate, as a heat dissipating layer at the upper layer or lower layer, or at the upper and lower layers, of the active layer of a thin film transistor.
However, Japanese Patent Application Laid-Open No. 2003-131588 does not at all mention the concrete relationship between the thermal conductivity of the substrate and the thermal conductivity of the heat dissipating layer, and it is unclear whether or not the film that has higher thermal conductivity than the glass substrate sufficiently achieves the function of a heat dissipating layer.
Thus, in a case in which an IGZO oxide semiconductor is used as an active layer, when the temperature at the time of heat generation becomes greater than or equal to 100° C., the minute amount of moisture that exists within the substrate or the active layer, the insulating layer or the electrodes is affected, and the so-called TFT characteristic is changed. Further, when the temperature at the time of heat generation becomes greater than or equal to 300° C., the oxygen that exists within the active layer changes, and the TFT characteristic is changed.
Further, in the fabrication of a flexible device that uses a resin substrate, because the thermal conductivity of the substrate is lower, it is predicted that it will be easy for heat to be stored and that the deterioration in driving that is due to heat generation will be more marked. Further, it is thought that such effects of heat generation are, at a resin substrate that has low heat resistance, a source that gives rise to changes in the dimensions of the substrate due to heat storage, and the like.
However, Japanese Patent Application Laid-Open No. 2003-131588 does not mention the above-described point whatsoever.